The predominate approach today to introduce factory automated technology into manufacturing is to selectively apply automation and to create islands of automation. The phrase "islands of automation" has been used to describe the transition from conventional or mechanical manufacturing to the automatic factory. Interestingly, some appear to use the phrase as though it were a worthy end object. On the contrary, the creation of such islands can be a major impediment to achieving an integrated factory.
Manufacturing examples of islands of automation often include numerically controlled machine tools; robots for assembly, inspection, painting, and welding; lasers for cutting, welding and finishing; sensors for test and inspection; automated storage/retrieval systems for storing work-in-process, tooling and supplies; smart carts, monorails, and conveyors for moving material from work station to work station; automated assembly equipment and flexible machining systems. Such islands are often purchased one at a time and justified economically by cost reductions.
To integrate the islands of automation it is necessary to link several machines together as a unit. For example, a machine center with robots for parts loading and unloading can best be tied to visual inspection systems for quality control. Computer numerical control machine tools can all be controlled by a computer that also schedules, dispatches, and collects data. Selecting which islands to link can be most efficiently pursued on the basis of cost, quality and cycle time benefits.
In some cases the islands of automation will be very small (e.g. an individual machine or work station). In other cases the islands might be department-sized.
From a systems viewpoint, islands of automation are not necessarily bad, so long as they are considered to be interim objectives in a phased implementation of an automated system. However, to obtain an integrated factory system, the islands of automation must be tied together or synchronized. Systems synchronization frequently occurs by way of a material-handling system; it physically builds bridges that join together the islands of automation.
Automated material handling has been called the backbone of the automated factory. Other than the computer itself, this function is considered by many automation specialists as the most important element in the entire scenario of automated manufacturing. It is the common link that binds together machines, workcells, and departments into a cohesive whole in the transformation of materials and components into finished products.
To date, the major application for industrial robots has been material handling. Included here are such tasks as machine loading and unloading; palletizing/depalletizing; stacking/unstacking; and general transfer of parts and materials--for example, between machines or between machines and conveyors.
Robots are often an essential ingredient in the implementation of Flexible Manufacturing Systems (FMS) and the automated factory. The automated factory also will include a variety of material transportation devices, ranging from driver-operated forklifts to sophisticated, computer-operated, real-time reporting with car-on-track systems and color graphics tracking. These material transport systems serve to integrate workcells into FMS installations and to tie such installations and other workcells together for total factory material transport control.
With all of their versatility, robots suffer from a limitation imposed by the relatively small size of their work envelope, requiring that part work fixtures and work-in-process be brought to the robot for processing. Complete integration of the robot into the flexible manufacturing system requires that parts and subassemblies be presented to the robot on an automated transport and interface system. For example, installation of a machine tool served by a robot without an automated transport system will result in an inefficient island of automation needing large stores of work-in-process inventory for support, which are necessary to compensate for the inefficiencies of manual and fork truck delivery.
Robots may load and unload workpieces, assemble them on the transport, inspect them in place or simply identify them. The kind of activity at the robot or machine and material transport system interface dictates the transport system design requirements. One of the design variables relating to the interface includes accuracy and repeatability of load positioning (in three planes). Also, care in orienting the workpiece when it is initially loaded onto the transport carrier will save time when the work is presented to the robot or the tool for processing. Proper orientation of the part permits automatic devices to find the part quickly without "looking" for it and wasting time each time it appears at the workstation.
Fixtures may be capable of holding different workpieces, reducing the investment required in tooling when processing more than one product or product style on the same system.
The transport system must be capable of working within the space limitations imposed by building and machinery configurations, yet must be capable of continuous operation with the loads applied by a combination of workpiece weight, fixture weight, and additional forces imposed by other equipment used in the process.
The system must also have the ability to provide queuing of parts at the workstation so that a continuous flow of work is maintained through the process. Automatic queuing of transport carriers should provide gentle accumulation without part or carrier damage.
Robots and the machine tools they serve usually make up a large part of the cost of implementing an automated factory. Part of the justification for the investment is obtained through the ability to increase the operating time of equipment within the plant. With FMS's, the objective is to have machine tools cutting metal for 80% of the time instead of the historic 30%. Therefore, it is important that the materials transport system serving the robots and machine tools be capable of quickly moving into position with parts for loading into the machine, then quickly moving out of the workstation and on to downstream stations. Prompt transporter movements between stations allow work-in-process inventory to be minimized. Batch sizes are smaller and work faster with only a minimum of queuing at each workstation.
Tool changing by robots as an alternative to dedicated, automatic tool changers is becoming attractive owing to flexibility and relative lower cost. A robot equipped with special grippers can handle a large variety of tools, and the tools can be shared quickly by several machines.
For example, a robot which is positioned on a riser may load and unload two identical vertical milling machines with one of many tools. All the tools are stored in a rack in front of the robot between the two milling machines. The parts being machined are brought to the milling machines on auto transfer devices. The entire operation is controlled by a host computer that directs the robot controller and signals it which part type is coming to the robot and which set of tools to select to load and unload into the milling heads.
In designing a transport system, a determination must be made of how the parts are to be presented to the robot and to the workstation, and whether special carrier-top pallet designs are required. It is sometimes possible to move the parts already held in the same fixture that will be utilized by the machine tools during machining. The transport carrier is designed to accommodate chuck-like fixtures which are transferred from the carrier by a robot mounted directly on the machine or turning center. Rapid exchange of parts facilitates the integration of tools and the transport system into a smoothly functioning FMS.
A link for tying together some of the independently automated manufacturing operations is the automatic guided vehicle system (AGVS). The AGVS is a relatively fast and reliable method for transporting materials, parts or equipment. Guide path flexibility and independent, distributed control make an AGVS an efficient means of horizontal transportation. As an alternative to traditional conveying methods, the AGVS provides manufacturing management with a centralized control capability over material movement. Information available from the AGVS also provides management with a production monitoring data base.
Robot installations for transporter interface can be grouped into three principal categories: (1) stationary robots, (2) moving robots (on the floor or overhead), and (3) robots integral with a machine. The moving robots subdivide into two types. First are stationary robots, mounted on a transporter to move between work positions to perform welding, inspection, and other tasks. The second type of moving robot is the gantry unit that can position workpieces weighing more than one ton above the workcells and transport system. The system only has to deliver and pick up somewhere under the span of gantry movement.
A gantry robot can be described as an overhead-mounted, rectilinear robot with a minimum of three degrees of freedom (DF) and normally not exceeding six DF. The robot is controlled by a multi-microprocessor controller allowing it to interact with a multitude of other devices.
Large work envelopes, heavy payloads, mobility, overhead mounting, and the capability and flexibility to do the work of several pedestal-mounted robots are some of the advantages of implementing a gantry robot versus a floor or pedestal-mounted robot.
Gantry robots have been around for many years in various forms, from refueling systems in the nuclear reactor cell to large material-handling systems in the mining industry. There are also pseudogantry robots which are composed of primarily a pedestal robot mounted in the inverted position and on slides, allowing it to traverse over the work area. Since gantry robots are somewhat unique, some terms are used that do not pertain to pedestal robots, as follows:
Superstructure: Also called the gantry support structure or box frame. This is the structure upon which the robot will be elevated from the floor. It is an integral and essential portion of a gantry robot system.
Runway: The longitudinal X axis of the gantry robot. It is normally the passive side rails of the superstructure.
Bridge: The transverse or Y axis of the gantry robot. The bridge is an active member of the robot riding on the runway rails and supporting the carriage.
Carriage: The support structure for the Z axis. Provides the Y axis motion on the bridge.
Telescoping tubes/masts: Depending on the robot this is the vertical or Z axis of the gantry robot. In the case of telescoping tubes, they come together, allowing for a lower ceiling. A sliding mast slides along its length up and down, requiring a ceiling height equal to its stroke above the superstructure.
There are two major designs of gantry-style robots, the four-poster and the cantilever. The four-poster gantry has a complete overhead structure which covers the entire work envelope. The robot is mounted much the same as an underslung bridge crane. The axes consist of an X, Y, and direct vertical Z in Cartesian coordinates with optional wrists that enable straightforward programming and control.
Some of the advantages of the four-poster gantry are: (1) the design can be very modular, allowing for a wide range of sizes in both the X and Y axes; (2) design modularity of supports can allow for heavy payloads; (3) a large work envelope can be provided at a very reasonable cost; and (4) the Cartesian coordinates allow for application of a variety of proven software schemes, including CNC-type controls.
The alternative gantry style is the cantilever type. The basic advantages of this type of robot include: (1) modularity of the X axis, allowing for very long travel; (2) the ability to apply a rotary wrist, making both sides of the gantry available as separate work spaces; (3) a programmable structure overhead, allowing clearance to load and unload parts from above using a crane or forklift, for example; (4) open accessibility from all directions, allowing conveyors, pallets, or part feeding from any direction; (5) design rigidity, permitting extreme accuracy and reliability for light machining tasks or routing applications; and (6) cartesian coordinates and rigid design combination, providing for application of a CNC-type controller with the inherent accuracy to permit off-line programming.
The gantry can be linked to advanced computer control because it offers simplicity of movement and high accuracy. With the system's off-line programming capability, a marriage can be made with CNC machine tools for automatic reprogramming, making small-batch automation economically feasible.
The cantilevered gantry can be used with direct numerical or hierarchical control. It can be coupled with communication and supervisory computers in FMS or complete factory automation systems. The gantry robot can also be fitted with vision and adaptive or advanced sensory interfaces to provide real-time path modifications.
Today's gantry robots have the capability of handling very heavy payloads. Heavier payloads sometimes require stronger end effectors. End effectors for gantry robots sometimes can become very complex, since they can perform more than one task. These end effectors can become very heavy, thereby reducing the effective payload.
End effectors used in material handling such as palletizing include all of the conventional styles--standard grippers, vacuum cups, electromagnets--and many special designs to accommodate unusual application requirements. Dual-purpose tooling is often used to pick up separators or trays, as well as the parts being moved through the system.
Vacuum-type grippers and electromagnetic grippers are advantageous, because they permit part acquisition from above rather than from the side. This avoids the clearance and spacing considerations that are often involved when using mechanical grippers.
However, the use of vacuum and electromagnetic grippers is not without its problems since cycle time is not just a function of robot speed and its accelerating/decelerating characteristics. Cycle time is dependent on how fast the robot can move without losing control of the load. Horizontal shear forces must be considered in the application of these grippers. This often means that the robot is run at something less than its top speed.